Ajantha's away spinning delivery to the right handed batsman is released with a Carrom like flick of the middle and index fingers, hence the term Carrom Ball.
Understanding the Carrom Ball
Carrom is a table-top game and the goal is similar to pool. The video here shows the use of the middle and index finger in Carrom.
Why is it hard to pick?
As opposed to the conventional spin bowler Ajantha releases the ball at a much quicker speed and as a result the batsman has little or no time to make an adjustment off the wicket.
The grip is similar to an off spin delivery which would turn into the right handed batsman but in this case the middle finger flicks the ball in the opposite direction at the point of release.